1. Field of the Invention
The present invention relates to an optical glass composition, a preform and an optical element. In particular, the present invention relates to: an optical glass composition suitable as the material of an optical element such as a lens element contained in a shooting lens system of a digital still camera or a digital video camera (simply referred to as a digital camera, hereinafter); a preform used in producing this optical element by press molding; and this optical element.
2. Description of the Background Art
In recent years, in digital cameras, various types ranging from common types to high-class types have been proposed in accordance with the consumers' needs. Among such digital cameras, in high-class type digital cameras particularly, high performances such as both of wide angle and high magnification are strongly desired as well as thickness reduction for improving portability.
In order to achieve thickness reduction in a digital camera, reduction is indispensable in the thickness of the shooting lens system which occupies a relatively large volume. For the purpose of thickness reduction in a shooting lens system, reduction in the number of lens elements is effective. Nevertheless, in recent years, increasingly high optical performance is required in shooting lens systems. Thus, reduction in the number of lens elements is reaching a limit for increasing magnification particularly, and hence further reduction is unexpectable. Accordingly, for the purpose of thickness reduction in a shooting lens system, thickness reduction becomes necessary in the individual lens elements contained in the shooting lens system.
For the purpose of thickness reduction in a lens element, it is effective to increase the refractive index of the glass material that forms the lens element. For simultaneously achieving the above-mentioned high performances such as both of wide angle and high magnification, it is generally preferable that the dispersion of the lens element is adjusted to relatively lower. As examples of such a glass material having a high refractive index and showing a relatively lower dispersion, optical glass is described in Japanese Laid-Open Patent Publication No. 2003-267748.
In a shooting lens system, it is expected that high performances such as both of wide angle and high magnification in addition to thickness reduction can be realized when a glass material having a high refractive index and showing a relatively lower dispersion is used, for example, for a most object side-negative lens element in a lens unit having negative optical power and also for a positive lens element in a lens unit located on the image side of the lens unit having negative optical power. Also, in an imaging lens system, aberration compensation is necessary. Then, in general, the aberration is compensated by variously combining the optical indices such as the refractive indices (nd) and the dispersions (vd: Abbe numbers) of the lens elements and the shapes of the lens elements. Thus, in order to achieve successful aberration compensation as well as both of thickness reduction and high performances in a shooting lens system, it is desired that for a lens element, a glass material having an Abbe number falling within a prescribed range relative to a high refractive index falling within a prescribed range is selected from glass materials having a high refractive index and showing a relatively lower dispersion. Simultaneously, it is desired that the optical power, the shape and the like of the lens element formed from such a glass material are adjusted to suitable ones, and that the lens element is located on a suitable position.
Japanese Laid-Open Patent Publication No. 2003-267748 discloses optical glass having optical indices such as a refractive index of 1.88 to 1.90 and an Abbe number of 35 to 50. In this optical glass, the refractive index and the Abbe number are defined by a prescribed condition. The optical glass has such a high refractive index. Nevertheless, the dispersion of the optical glass falls within wide range from lower to higher. In order to achieve successful aberration compensation as well as both of thickness reduction and high performances in a shooting lens system, such a lens element is needed as a lens element formed from, for example, a glass material having an Abbe number of approximately 45 relative to a high refractive index of approximately 1.85. Nevertheless, Japanese Laid-Open Patent Publication No. 2003-267748 does not specifically describe a glass material having the above-mentioned specific combination of the refractive index with the Abbe number, and showing excellent melting property, processability (droplet property), crystallinity and the like.